Recent increases in information volume have led to an increased need for techniques for storing information at high speed and capacity. The recording density of magnetic storage, the most widely used form of information recording, is currently approaching the theoretical limit, and a recording density of 1 Tbit/inch2 is considered to be the limit even when perpendicular magnetic recording is used. On the other hand, ferroelectrics have spontaneous polarization, and the polarization direction thereof can be reversed by applying an electric field from the outside. Consequently, digital data can be correlated to the orientation of this polarization in order to record information. It is widely known that the domain walls of a ferroelectric are only one to two lattice units wide and are markedly thinner than those of a ferromagnet, and because the domain size of a ferroelectric is also much smaller than the domain size of a ferromagnet, the ability to artificially control the extremely minute domains of a ferroelectric would make it possible to obtain an information recording element having extremely high density. However, the polarization in a ferroelectric material is masked by electrons, ions, and other surface charges adhering to the surface of the material, the polarization has been difficult to measure; i.e., recorded information has been difficult to read.
An SNDM (scanning nonlinear dielectric microscope) is known as a device for detecting the polarization distribution of a ferroelectric by a purely electrical method. FIG. 1 is a block diagram showing a publicly known device for detecting the polarization direction of a ferroelectric using an SNDM. This device determines the polarization direction of a ferroelectric by measuring the nonlinear permittivity of a ferroelectric material, i.e., the capacitance Cp directly below the probe 3. In this device, in order to detect the polarization direction of the ferroelectric material 1, an alternating electric field is applied from the outside between a stage 2 and a ring probe 4 and probe 3. The oscillation frequency of an oscillator 5 is then varied in accordance with the alternating electric field. Since the ratio of variation of oscillation frequency, which includes the polarity, at this time is determined by the nonlinear permittivity, i.e., the capacitance Cp directly below the probe, the polarization distribution of the ferroelectric material is detected by two-dimensional scanning of the ratio of the frequency variation by the probe 3. Specifically, after the frequency variation of the oscillator 5 is demodulated by an FM demodulator 6, the polarization distribution is detected by synchronous detection at the frequency of the applied electric field by a lock-in amplifier 7.    Patent Document 1: Japanese Laid-open Patent Publication No. 2004-127489